Sensor devices

ABSTRACT

Sensor devices for examining fluid samples having, between the sample under examination and a detector, a membrane made of polyvinyl chloride in un-plasticized form. This membrane material acts as a barrier to paracetamol and sugars but is permeable to hydrogen peroxide and to oxalate, so these species can be determined after passing through the membrane. When they are not present as such in the sample, they may be formed from analytes which are sought (e.g. by enzymic action, which is especially applicable to the determination of glucose, using an oxidase enzyme). The membranes may be made by solvent casting, conveniently to a thickness of 10 to 40 μm, and used in electrolytic systems with a platinum anode. The membrane material can be formed into multi-layer membranes, which may incorporate layers of material to protect it or modify the permeability, or added materials for example enzymes.

This invention relates to sensor devices such as are used in thedetermination of a component or components which may be present in afluid sample, such as a physiological fluid (e.g. blood) or other fluidsof biological origin (e.g. fruit), process fluids or effluents.

Many forms of sensor have been proposed, and commonly these rely on someform of membrane to control the extent to which the components presentin a sample under examination can gain access to an electrode, at whichthey can then be detected and determined. Especially, it is well knownto make sensors using membranes to separate the media being analyzedfrom the active electrode itself. The main function of the membrane isto separate, as far as possible, those components which are desirable(i.e. can take part in the reactions at the electrode on which thedesired determination depends) from interferents (i.e. compounds whichmay be present but are undesirable because they either interfere withthe progress of the desired determination reactions or take part inreactions of their own which compete with those of the component soughtand distort or overwhelm the signals which are to be measured. The formsof construction have much in common with each other, and mainly differin the nature of the membrane or media within it or combined with it insome way.

Some forms of sensor rely on the components used to make the membrane,while others rely on the mode of fabrication of the membrane, selectingits physical properties (for example its porosity) or treatments givento it, as these factors can control its effectiveness and selectivity inuse.

Other forms of sensors incorporate an enzyme, which converts onesubstrate compound or analyte into another which may then be more easilymeasured. Especially, it is known to use oxidase enzymes, which generatehydrogen peroxide--a substance which can be measured very convenientlyand very accurately by electrolytic methods, especiallyamperometrically. An example is European Patent No. 216577 (of ICI PLC)which specifies an enzyme electrode sensor with membrane barrier of low(>5%) porosity.

Materials which have been proposed for the fabrication of membranes forsensors include polyvinyl chloride (PVC), but only in plasticized form,as described in European Patent Application No. 92302131.5, where theplasticizer performs a necessary part of the function of the membrane.

Such known sensors, utilising various membrane materials, are veryvaluable in analytical techniques and the analysis of biological fluids(e.g. blood) for the presence of substances considered critical formedical reasons--e.g. glucose and other materials which may beoxidizable or reducable, or have toxic properties.

We have now found that the selectivity of such a membrane-enclosedsensor can be significantly and surprisingly modified and improved bymaking the membrane of polyvinyl chloride (PVC) itself, inun-plasticized form.

Thus according to our invention we provide an improved sensor devicecomprising means for detecting components present in fluid samples andproviding an output representative of the content of said componentcomprising a detecting means and a membrane barrier between thedetecting means and the sample to be analyzed, characterised in that themembrane barrier is composed of polyvinyl chloride (PVC) itself, inun-plasticized form.

The detecting means is most conveniently of one of an electrochemicalnature, but other types (e.g. spectrophotometric or optical detectingsystems) may be used if desired. The detecting means will usuallycomprise an electrode system and a liquid or gel phaseelectrolyte-containing medium. In most applications the electrolyte willbe aqueous (i.e. aqueous or aqueous-based) but the use of non-aqueouselectrolyte media (for example organic-based media) is not excluded.

According to our invention we also provide a method for determining acomponent in a fluid sample, which comprises contacting the sample witha sensor device as defined above.

Especially, in the devices and method of our invention, the sensordevice comprises a detecting means in contact with an electrolyte mediumand both associated with a membrane of polyvinyl chloride (PVC) itself,in un-plasticized form, which provides an interface for contact with asample to be analyzed and interposed between the active electrode(anode) of the cell used as detector. Alternative forms of constructionmay have both the electrodes of the detecting means (cell) enclosedwithin the PVC membrane (so that both are separated from the sample) or(b) only the anode of the detecting means (cell) enclosed within the PVCmembrane (so that the PVC membrane is between the electrodes and thecathode is not separated from the sample). Of these, the former is veryconvenient and compact, but the latter is more simple and is madepracticable by the fact that PVC in un-plasticized form is lessinsulating in its properties, and H⁺ ions can pass through it.

The sensor device of our invention can have a single membrane or, ifdesired, multiple layers of membrane material. When multiple layers ofmembrane are used, these may be the same or different. The preferencefor the position to be occupied by the un-plasticized PVC membranediffers to some degree according to the particular use to which thesensor is being applied and what substrate compound is to be detected bythe sensor.

The governing factor is mainly the fact that un-plasticized PVC ispermeable to hydrogen peroxide and some low molecular weight species(for example oxalate) but impermeable to some sugars, and permeable toothers for example glucose. Thus, when the sensor is to be used fordetecting a species to which the PVC is impermeable, a sensor assemblyis best made with an enzyme (with which the desired substrate caninteract and generate a species which can pass through theun-plasticized PVC) situated outside the un-plasticized PVC membrane, sothat the desired species in the sample under examination can makecontact with the enzyme--either directly or indirectly, through anotherselective membrane layer--and thereby generate a species to which theun-plasticized PVC membrane is permeable. Especially, when glucose isthe species sought, an oxidase enzyme can be used and the hydrogenperoxide thus generated can pass through the un-plasticized membrane andbe determined at the detector means.

When multiple membrane layers are used, any membrane layer or layersother than any comprised of un-plasticized PVC may be made of any of thewide variety of materials known in the art. Examples of these includedialysis membranes, and in general are preferably non-diffusion limitingmembranes, at least to the extent that they do not limit diffusion andpassage of desired species towards the detecting means. Whenun-plasticized PVC is used as in inner membrane (i.e. a layer which isnot the outermost), then one or more outer layers may be used which areof material which protects the sensor assembly in a mechanical manner(e.g. from mechanical damage) or in a chemical or any other mannerconsidered appropriate for the use to which it is to be applied. Thusfor example, there may be used an outer layer comprising a polycarbonate(especially in a porous form).

The active electrode may be any of those known in the art, for example ametal electrode, but especially a platinum anode. This is mostconveniently made in combination with a silver/silver chloride counterelectrode, as for example in the so-called Clark electrode, whichcomprises a platinum electrode surrounded by a silver/silver chloridering.

The PVC (polyvinyl chloride) may, be any polymer of vinyl chloride, asfor example those made and available commercially, but should be freefrom any added plasticizer (an ingredient which is often present in somecommercial products intended for uses such as molding). Such"un-plasticized" PVC polymers are readily obtainable in commerce,however, and it is necessary only for the quality and purity of anypolymer to be checked, whether by its specification or labelling. Themolecular weight of the PVC is relatively non-critical, and mostcommercial grades will be satisfactory in use. A typical molecularweight is in the range 10,000 to 200,000, but others may be used ifdesired.

The material (i.e. the un-plasticized polyvinyl chloride) may be madeinto membranes by any conventional method. Most conveniently, this canbe done by solution casting techniques, using solvents to dissolve thepolymer and then spreading the solution on a plate or flat surface andallowing the solvent to evaporate. A convenient solvent istetrahydrofuran (THF), but other solvents or mixtures of solvents whichare known to be able to dissolve PVC be used if desired.

The thickness of the membranes can be of the order already usedconventionally in the art, but my be varied as found most appropriatehaving regard for the particular mixed polymer composition being usedand the conditions under which it is to be used. Thus a convenientthickness is in the range 10 to 40 μm, though larger or smallerthicknesses can be used if desired.

There are two principal forms of construction which may be used tosecure the advantages of the new membrane material we now propose. Inone, the un-plasticized PVC is the outer membrane, and in the other itis the inner membrane.

The forms using an enzyme will have basically the constructionsequences: ##STR1##

For these, the components (apart from the un-plasticized PVC membrane)are mainly the conventional ones, and the many variants known in the artmay be used.

Un-plasticized PVC is not sufficiently permeable to glucose, so directelectrochemical detection of glucose cannot be achieved through thismaterial as membrane. However, it is permeable to hydrogen peroxide, soenabling an alternative glucose-permeable membrane and an oxidase enzymeto be used to produce hydrogen peroxide, which then can selectivelypermeate the un-plasticized PVC membrane.

This un-plasticized PVC membrane has a particular advantage overplasticized PVC in that it is (unlike the plasticized form) impermeableto paracetamol--so that this compound is excluded from interfering withhydrogen peroxide detection, as well as ascorbate/urate. Thisselectivity against paracetamol and the like while in favor of hydrogenperoxide is a valuable property which is not easily found and it veryuseful in clinical and related analytical, diagnostic and monitoringwork.

Consequently, any other sensor device or system using an oxidase enzymeto generate hydrogen peroxide can, with advantage, incorporate anun-plasticized PVC membrane as part of its construction, especially asan inner membrane near to the active electrode surface.

Another advantage is that sensor devices of the electrolytic(amperometric) type are often used for examination of biological fluidsfor example blood, urine, and the like. It happens that urine contains anumber of interferents which--even though their identity has not yetbeen established--have the ability to permeate many of the knownconventional membrane materials, for example plasticized PVC, and sointerfere severely with the accurate estimation of components such ashydrogen peroxide and of compounds which can be used to generate it. Theun-plasticized form of PVC does not allow these unidentified componentsto pass through, is it constitutes an effective barrier to them andfacilitates the wider applicability and accuracy of measurement. Thus,for many applications the un-plasticized PVC membrane will bepreferable, though it will not always be so in applications in which itis desired to have a phenolic compound permeating the membrane.

Another use for the un-plasticized PVC membrane is in the constructionof a sensor device for the determination of oxalate. It acts in this intwo ways, both of which are valuable. One is a form of sensor in whichthe un-plasticized PVC membrane is used as an inner membrane beneath alayer or membrane comprising an oxalate oxidase system, so that hydrogenperoxide is generated from the sample and the resulting hydrogenperoxide passes through the un-plasticized PVC membrane; this relies onits action as a membrane selectively permeable to hydrogen peroxide.

Alternatively, the un-plasticized PVC membrane can be used as an outerlayer with the means for oxalate detection beneath it; this relies onits unexpected ability to act as a membrane selectively permeable tooxalate. Un-plasticized PVC is the only material that we know of atpresent which is oxalate-selective, and the mechanism by which itperforms in this way is still not clear to us. In this mode orembodiment of our invention, the means for detecting the oxalate whichhas passed through the un-plasticized PVC membrane may be any of thoseknown and may be either an enzyme one (oxalate oxidase based) or onewhich detects and measures the oxalate directly by electrolytic action.This may be amperometric, and the pH of the medium around the activeelectrode may be adjusted as may be found most appropriate by simpletrial.

Whatever the mechanism by which the permeability to oxalate takes place,the ability of an un-plasticized PVC membrane to act so as to excludeserum and urine interferents from the active electrode region of asensor is true and highly effective and a valuable feature of thisinvention.

An example of a basic construction sequence utilizing thisun-plasticized PVC membrane can be represented as follows: ##STR2##

In this, oxalate or oxalic acid from sample (1) passes through theun-plasticized PVC membrane (2) and then, by contact with the enzyme(3), generates hydrogen peroxide and carbon dioxide. The hydrogenperoxide then passes through the inner membrane (4) to the electrode(5), at which it is determined electrolytically.

The inner membrane (4) may be of any convenient material--for example adialysis membrane or a polycarbonate membrane--intended to perform suchactions as regulation of the flow of components to electrode (5) (shouldthis be considered desirable) or to provide mechanical protection andadd to the robustness of the sensor assembly and minimize risk of damageto the innermost electrode elements.

Thus according to our invention we also provide, as useful new products,membranes comprising un-plasticized polyvinyl chloride. Especially, weprovide these in the form of multi-layer membrane products, in which atleast one layer is formed of un-plasticized polyvinyl chloride and iscombined with one or more layers of other materials of appropriateproperties to enhance the properties of the un-plasticized polyvinylchloride itself. Such materials may be of appropriate permeability toregulate the access of components before or after passage through theun-plasticized polyvinyl chloride, and/or of a physical form or strengthwhich protects the un-plasticized polyvinyl chloride from damage orprovides it with any desired degree of stability of shape or positioningin use.

Especially, we provide membranes of un-plasticized polyvinyl chlorideincorporating an enzyme, for example an oxidase. In these, the enzymemay be immobilized by the chemical means known in the art, or they maybe held between the layers of a multi-layer structure.

We claim:
 1. A sensor device comprising means for detecting componentspresent in fluid samples and providing an output representative of thecontent of said component, comprising a detecting means and a membranebarrier between the detecting means and the sample to be analyzed,characterized in that the membrane barrier is composed of polyvinylchloride (PVC) itself, in un-plasticized form and being selectivelypermeable to hydrogen peroxide, oxalate and oxalic acid and impermeableto paracetamol.
 2. A sensor device as claimed in claim 1 wherein thedetecting means comprises an electrolytic sensor.
 3. A sensor device asclaimed in claim 1 or claim 2 which comprises a detecting means incontact with an electrolyte medium which is in contact with a membraneof polyvinyl chloride (PVC) itself, in un-plasticized form, whichprovides an interface for contact with a sample to be analyzed andinterposed between the active electrode of the cell used as detector. 4.A sensor device as claimed in claim 3 wherein the electrolyte medium iscontained in a liquid or gel phase.
 5. A sensor device as claimed inclaim 3 wherein an enzyme is placed between the sample to be analyzedand the membrane to convert a component or analyte to be determined inthe sample into another compound which passes through the membrane andis then detected and measured by the detecting means.
 6. A sensor deviceas claimed in claim 5 for detecting a species to which un-plasticizedpolyvinyl chloride is impermeable, wherein the enzyme is located betweenthe un-plasticized polyvinyl chloride membrane and sample to beanalyzed, so that the desired species in the sample under examinationfirst contacts the enzyme and reacts to generate a species which passesthrough the un-plasticized polyvinyl chloride membrane to the detectingmeans.
 7. A sensor device as claimed in claim 6 wherein the enzyme is anoxidase, which is used to interact with an oxidizable component togenerate hydrogen peroxide--to which un-plasticized polyvinyl chlorideis permeable.
 8. A sensor device as claimed in claim 5 for detecting aspecies to which the un-plasticized polyvinyl chloride is permeable,wherein the enzyme is located within the un-plasticized polyvinylchloride membrane, so that the desired species in the sample underexamination first passes through the un-plasticized polyvinyl chloridemembrane and then makes contact with the enzyme and reacts to generate aspecies to which the detecting means is responsive.
 9. A sensor deviceas claimed in claim 1 or claim 2 comprising (1) a detecting means, (2) amembrane of un-plasticized PVC and (3) an electrolyte medium positionedbetween and in contact with (1) and (2), and with membrane (2) formingan interface with a sample to be analyzed.
 10. A sensor device accordingto claim 9 wherein the detecting means is an active electrode.
 11. Asensor device according to claim 10 wherein the active electrode is ananode.
 12. A sensor device as claimed in claim 9 wherein the electrolytemedium is contained in an aqueous-based liquid or gel phase.
 13. Asensor device as claimed in claim 1, wherein the polyvinyl chloride hasa molecular weight in the range 10,000 to 200,000.
 14. A sensor deviceas claimed in claim 1, wherein the polyvinyl chloride membrane has athickness in the range 10 to 40 μm.
 15. A sensor device as claimed inclaim 1, wherein the polyvinyl chloride membrane is made by solutioncasting.
 16. A sensor device as claimed in claim 1, wherein the activeelectrode of the detecting means is made of platinum.
 17. A sensordevice as claimed in claim 1, wherein the membrane of un-plasticizedpolyvinyl chloride is used as an inner membrane in conjunction with oneor more outer layers of material which protects the sensor assembly. 18.Method for determining a component in a fluid sample, which comprisesusing a sensor device as claimed in claim
 1. 19. Method as claimed inclaim 18 wherein the component to be determined is hydrogen peroxidewhich is determined either directly by being present as such orindirectly by being generated from another component or analyte. 20.Method as claimed in claim 18 wherein the component to be determined isoxalate and which is determined either directly as such or as anothercompound generated therefrom.
 21. Method as claimed in claim 20 whereinthe means for detecting the oxalate which has passed through theun-plasticized polyvinyl chloride membrane is either an enzymic one orone which detects and measures the oxalate directly by electrolyticaction.
 22. Method as claimed in claim 18, employed for the study ofbiological fluids.
 23. A multiple-layer membrane product, comprising atleast one membrane layer composed of un-plasticized polyvinyl chloridewhich is selectively permeable to hydrogen peroxide, oxalate and oxalicacid and impermeable to paracetamol and an enzyme for converting oneanalyte component into another which is more readily determined at adetector means.
 24. A membrane according to claim 23 wherein the enzymeis an oxidase.